The subject matter described herein relates generally to electric power converters, and more specifically, to decreasing current imbalances within electric power converters.
Generally, a wind turbine includes a rotor that includes a rotatable hub assembly having multiple blades. The blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. At least some of the known wind turbines are physically nested together in a common geographical region to form a wind turbine farm. Variable speed operation of the wind turbine facilitates enhanced capture of energy when compared to a constant speed operation of the wind turbine. However, variable speed operation of the wind turbine produces electric power having varying voltage and/or frequency. More specifically, the frequency of the electric power generated by the variable speed wind turbine is proportional to the speed of rotation of the rotor. A power converter may be coupled between the wind turbine's electric generator and an electric utility grid. The power converter receives electric power from the wind turbine generator and transmits electricity having a regulated voltage and frequency for further transmission to the utility grid via a transformer. The transformer may be coupled to a plurality of power converters associated with the wind turbine farm.
Many known power converters include known semiconductor-based portions for electric power conversion, e.g., rectification portions and inverter portions. Most known rectification portions are used for converting alternating current (AC) to direct current (DC) and most known inverter portions are used for converting DC current to AC current. Many known semiconductor-based portions include power bridge circuits that include a plurality of power semiconductor switches, e.g., insulated gate bipolar transistors (IGBTs). The IGBTs are coupled in an electrically parallel configuration within an IGBT module. Also, a plurality of IGBT modules are coupled in an electrically parallel configuration within the rectification and inverter portions to increase the power ratings and reliability of the power converters. The IGBT modules are grouped by phase.
However, when IGBT modules are coupled in parallel, the electric currents transmitted through the modules do not balance evenly. The current imbalances are partially facilitated by electromagnetic fields induced by the currents conducted therethrough. The magnitude of the field strength is proportional to the magnitude of the electric current conducted through each IGBT module. The current imbalances are further facilitated by the interaction of transient current sharing features of the IGBTs and their associated circuits including different transfer characteristics of the IGBTs, for example, different gate threshold voltages. The IGBT with the lowest threshold voltage will turn-on and conduct first and turn-off last, thereby conducting longer than the other IGBTs. Such different conduction periods induce different temperature values in the IGBTs that induce different current values, different switching losses, and different thermal stresses while conducting in the on-state. Moreover, variations in stray inductances associated with IGBT emitters in different parallel IGBTs facilitate increasing such variations, i.e., imbalances in electric current conducted therethrough. Such current imbalances are typically manifested as higher currents on outside branches of IGBTs as compared to inside branches of IGBTs. In such instances, the highest stressed IGBT limits the power converter output, such that the lower stressed IGBTs do not conduct at their rated capabilities. Therefore, the outside branches attain current values close to their current ratings while the interior branches have remaining capacity, thereby limiting the total current flow through the power converter.
Alterations to individual IGBT circuits to compensate for current imbalances may provide some correction on a small scale. Examples of such circuit alterations include physically positioning conductors electrically coupled in parallel in transposed configurations. However, such small-scale solutions may not sufficiently affect current imbalances on larger scale devices, such as those associated with electric power converters for large generation facilities and large drive devices. Also, current balancing, or sharing, solutions for high current, e.g., in excess of 1000 amperes (amps), and lower frequency, e.g., 50/60 Hertz (Hz) devices, such as bus bars, are known. However, semi-conductor devices such as those used in electric power converters operate at lower currents, e.g., less than 1000 amps, and at higher frequencies, e.g., greater than 100 Hz. As such, greater power conversion capacity will necessitate either a greater number of power converters or larger power converters, both options requiring increased capital and operational costs.